The swirl sign is a radiological sign of acute extravasation of blood into a hematoma on non-contrast CT. It represents unclotted fresh blood which has lower attenuation compared to clotted blood, and this is suggestive of active bleeding. 

Active bleeding gives the same appearance of contrast extravasation on CT angiogram but additionally contrast extravasation may be seen as the CTA spot sign.

Special thanks to Dr Tamer Elholiby.